This application claims priority of Japanese Patent application number 2000-396619, filed Dec. 27, 2000.
This invention relates to a dispersion compensator and more specifically to a dispersion compensator to compensate group velocity dispersion and dispersion slopes in optical communications.
Currently, the research of large capacity optical communication networks using wavelength division multiplexing (WDM) transmission systems has been eagerly performed according to the plan to use them as infrastructures for supporting the future information-oriented society. At the same time, dispersion compensating devices to greatly improve the performance of the optical networks have become extremely important. They are also researched and examined.
As one of conventional dispersion compensators, a configuration to combine an optical circulator and a dispersion compensating fiber is well known. See U.S. Pat. Nos. 5,701,188, 5,715,265 and 5,974,206, and Japanese Laid-Open Patent Publication No. Heisei 8-316912 (i.e. U.S. Pat. No. 6,055,081).
A dispersion compensating fiber is a fiber having a chromatic dispersion value of a sign reverse to that of an optical transmission fiber and preferably its dispersion slope inclines reversely to that of the optical transmission fiber. The dispersion compensating fiber also can be composed of a fiber that forms a chirped grating. Although the optical circulator is generally used as a means to separate input light and output light, other configurations such as to use Y branch waveguide (See Japanese Laid-Open Patent Publication No. 2000-235170) and to use a 3 dB optical coupler (see Japanese Laid-Open Publication No. Heisei 11-331075) are also known. To broaden a band, a configuration to connect a plurality of the above-described basic configurations as a multistage type has been proposed.
When an optical fiber grating is practically to be applied to optical communications, its length needs to be one meter or more. It is difficult to form such a long grating in an optical fiber all at once. Accordingly, it is necessary to form gratings separately on a plurality of areas. However, this method requires high production costs.
In the conventional configuration to use a Y branch waveguide instead of an optical circulator, to keep the phase relation between two branched light waves is too difficult to realize.
In the conventional configuration to use an optical coupler instead of an optical circulator, it is difficult to realize a predetermined phase relation between the two light waves that are branched and fed back.
A dispersion compensator comprises a first optical path pair having a first and a second optical paths, a second optical path pair having a third and a fourth optical paths, an optical coupler connecting to one end of the first, second, third, and fourth optical paths to split the light from the first optical path pair to the third and fourth optical paths and the light from the second optical path pair to the first and second optical paths, an optical signal input/output device to input the optical signal into the first optical path through the other end of the first optical path and to output the light output from the other end of the first optical path, and three mirrors respectively disposed on the other end of the second, third, and fourth optical paths. With this configuration, the chromatic dispersion can be compensated with a compact configuration.
For example, the optical coupler is composed of a beam splitter and the first, second, third, and fourth optical paths consisting of an open optical system respectively. Preferably, at least two out of the three mirrors are slidable in the direction of respective optical axis. With this configuration, the wavelength characteristics of the dispersion compensation can be controlled.
The optical coupler also can include a directional optical coupler. In this case, the first, second, third, and fourth optical paths consist of an optical waveguide respectively. With this configuration, a compact and stable dispersion compensator is realized. Preferably, means to control optical lengths of at least two of the first, second, third, and fourth light paths should be disposed. With this configuration, the wavelength characteristics of the dispersion compensation can be controlled.
By using an optical coupler which optical branching ratio depends on a wavelength, the wavelength characteristics of group velocity dispersion can be controlled.